First off, a couple of honorable mentions - Shadows over Camelot, Marvel Legendary and Catacombs. All of these games catch the eye as something a bit different.
#7 - Tsuro - As my review showed, Tsuro is truly a piece of art. The board looks amazing, and the game is simple enough to explain to passersby quickly. Players have time between their turns to talk, and the game creates a lighthearted atmosphere that encourages people watching.
#6 - Battlestar Galactica - This is a game that draws people in based on the table talk. It will catch the attention of people familiar with the TV show. The game has enough tension and deduction that outside people can try to figure out who's a cylon and who's a human.
#5 - Star Trek: Fleet Captains - Fans of Star Trek will gravitate towards this game. The ships and crew cards evoke a great sense of Star Trek. The game is gorgeous, especially if one paints the ships. It's still a game that makes people ask "what's that?" The game isn't perfect, but it is eye catching and interesting enough for other people to enjoy.
#4 - Werewolf - While Werewolf isn't the most fun game to watch, it is interesting to see how people behave in different situations. It's fun to see how Werewolves try to avoid lynching, and it's also enjoyable to listen to a good moderator keep the game moving while telling an interesting tale.
#3 - Resistance - Much like Werewolf, Resistance is a game where the players are going to make wild accusations while trying to work together. No one is revealing their plans publicly, but everyone has an agenda. I've had several people watch a game of Resistance and then jump in because they had such a good time.
#2 - King of Tokyo - Dice and monsters seem to make watching a game very easy. I haven't had people stay and watch this game, but I have had a lot of people want to play if they catch us near the end of a game. People will stop and ask some questions so they know what's going on, so that much is good.
#1 - X-Wing - I don't need to gush even more about X-Wing, but the facts are that the models are stunning, the game looks like a lot of fun, plus it's Star Wars. Those 3 things put together make people stop every time I play this in public. They'll watch a round, maybe even a few, but without fail, someone will want to learn more about this game. I've added a few things that help, mainly the black felt mat, but even without that, the game stands on its own.